UV-stable grease or seal substance for optical systems, manufacture thereof as well as use thereof

ABSTRACT

A UV-stable lubricating and/or sealing compound, and method for production thereof. The compound includes a base oil and a powder-form inorganic thickener distributed homogeneously therein. At room temperature, the base oil has a vapor pressure of &lt;10 −10  mbar, the thickener includes at least one inorganic dry lubricant and/or ceramic filler with a maximum mean particle size of 10 μm.

The invention concerns a UV-stable lubricating and/or sealing compound, in particular for use under clean room conditions and/or in high vacuum and/or in contact with ultrapure gases for optical systems, for example, lithography optics, as well as a method for its production and its use.

The danger exists that undesired gases, such as, e.g., air, etc. can penetrate at contact and/or connection sites of components and mechanisms in devices that are placed under vacuum. For this reason, these sites or contact surfaces are frequently provided with a sealing compound, whereby possible irregularities in the contact surfaces will be filled. This will prevent the penetration of gases and the resultant decrease in vacuum. This type of vacuum is also contained in objectives, for example, so that their lenses are also provided with such a sealing paste in their mounts or adjusting rings.

Optical systems for lithography usually are comprised of a lithographic objective amd an illumination system. Within one objective, it is necessary to seal the individual mount components by introducing suitable means for sealing against the ambient atmosphere and/or against flushing gases. In addition, it is necessary, in particular, to lubricate movable components, such as, e.g., spindle adjusting drives, in illumination systems. In addition to optical systems for lithography, which are operated at atmospheric pressure, there are also imaging methods which require high vacuum conditions (for example: x-ray lithography, electron beam lithography). Under the latter conditions, it is necessary to use sealing means which are additionally suitable for high vacuum.

Thus, lubricating and sealing compounds, such as, e.g., pastes, are suitable for lithography optics, but they must fulfill the following minimum requirements:

-   -   the vapor pressure will amount to <10⁻¹⁰ mbar at 20° C.     -   gaseous components that may lead to disrupting impurities, in         particular, if they might coat optical surfaces, should not be         discharged within lithography systems, since the optical         function will be negatively influenced;     -   they must be sufficiently stable relative to UV light, in         particular, light of wavelengths of less than 250 nm;

they must be sufficiently inert relative to materials and media of optical systems, in particular, lithography systems.

Such lubricating and sealing pastes usually are comprised of a base oil, in which finely pulverized solids are worked in as thickeners.

Basically, highly refined hydrocarbons, synthetic hydrocarbons, specific esters, pentaphenyl ether and particularly perfluoro polyethers are well suitable as the base oils. Less suitable are silica oils, since these fluids can lead to considerable interferences in lithography processes.

Lubricating and sealing pastes based on perfluoro polyethers are frequently thickened with PTFE powder. It has been shown that PTFE powder is no longer sufficiently stable against energy-rich stray radiation at wavelengths of less than 250 nm, such as may occur, e.g., in lithography systems. The breakdown products formed in this way frequently deposit on functional optical surfaces in the system, which leads to a reduced transmission and above all, to an increased reflection.

The object of the invention is thus to produce a lubricating and sealing paste, which does not break down, or only slightly decomposes even in lithography optics under light irradiation, in particular, in short-wave UV light, especially in UV laser light. The object of the invention is also to provide a compound, which has low rates of gas discharge. Finally, another object of the invention is to overcome the disadvantages of the prior art and to provide an improved lubricating and sealing means, which can also be used for movable mechanical parts.

This object is now achieved by the lubricating and sealing compound, which is defined in the claims.

It has been found, namely, that the above-described problem can be overcome by means of a lubricating and sealing compound that contains a base oil which has a vapor pressure of <10⁻¹⁰ mbar at room temperature. The base oil is preferably selected from the substance group of perfluoro polyethers.

The lubricating and sealing paste contains as a thickener finely dispersed solids with an average particle size of <10 μm. Preferably, the average particle size is the mean particle size d₅₀. The particle size determination is made by laser diffraction (CILAS laser granulometer). For nanoparticles (<100 nm), acoustic spectrometry is utilized for particle size analysis. In the characterization of fillers, in addition to particle size distribution (volume distribution), the (mean) particle size d₅₀ is most often used. The value d₅₀ is the particle diameter which is obtained when 50% of all measured particles are included in the cumulative representation. Powders with these data can be commercially obtained, e.g., from Wacker Ceramics, Kempten, Del., and Degussa Hüls. The solid involves an inorganic dry lubricant and/or a ceramic filler of high radiation resistance.

Volatile impurities are usually removed from the base oil used according to the invention prior to its use. This removal is preferably conducted by means of vacuum distillation, particularly high-vacuum distillation, wherein flash evaporators and thin-film evaporators are preferred, as well as by high-vacuum degassing. Readily volatile admixtures as well as those of average volatility and even those of difficult volatility can be removed in this way, such components usually being contained in the base oil as a result of the production process.

In a preferred embodiment according to the invention, the base oil used has a vapor pressure of <10⁻¹⁰ , in particular <10⁻¹², wherein a vapor pressure of <10⁻¹⁵, in particular, 10⁻¹⁶ mbar at 20° C. is most particularly preferred.

The determination of the gases discharged from the base oil utilized is preferably conducted by GC/MS analysis. In this way, volatile components of a sample are expelled by thermodesorption, concentrated in a cooling trap, and then introduced into a capillary colum of a gas chromatograph as the “sample”. In a mass spectrometer connected downstream, which serves as a highly sensitive detector, the individual substances of the expelled gaseous components of the sample are identified and quantitatively determined. In the GC/MS analysis of PFPE oils according to this invention, thermodesorption takes place at 200° C. for 2 minutes.

The thickeners utilized according to the invention are inorganic dry lubricants, such as metals, metal oxides, ceramic materials as well as metal salts. The dry lubricants utilized according to the invention preferably comprise BN (hexagonal structure), graphite, MoS₂, WS₂, NbS₂, TaS₂, AsSbS₄, AsAsS₄, WSe₂, NbSe₂, TaSe₂.

In another preferred embodiment, the compound according to the invention has inorganic fillers as the absorption agent for high-intensity UV radiation, as may occur in lithographic systems. Such fillers include, for example, ZrO₂, Ta₂O₅, HfO₂, TiO₂, SnO₂ and Nb₂O₅ as well as SiO₂ and Al₂O₃. Other metal oxides are, for example, cerium dioxide, as well as oxides of rare earths.

The compound according to the invention may contain additional additives, such as, for example, corrosion inhibitors or fluorescent markers, depending on the field of application. Fluorescent pigments based on rare-earth oxides are suitable as fluorescent markers.

The mean particle size of the thickener and fillers utilized amounts to <10 μm, preferably <5 μm, and most preferred <2 μm.

Preferably, the mean diameter of the finely distributed powder, as well as the other, optionally added solids, is a size of <100 nm, in particular <50 nm, whereby <20 nm and in particular <15 nm is most particularly preferred.

The compound according to the invention is produced in such a way that the thickener and solids are premixed in the base oil. After this, the compound is homogenized by dispersing the added solids. In this way, the crude paste is most intensively homogenized in order to break up agglomerates of particles, so that the particles are present individually finely distributed in the compound without forming agglomerates in the sealing compound. Such a homogenizing is possible, for example, by means of a three-roll mill, wherein the premixed crude paste is drawn between opposite-running rolls. In this way, the solid particles are subjected to an intense shearing and are simultaneously distributed uniformly in the compound or paste. The crude paste obtained in this way can then be mixed or extended with additional base oil or other, in particular, liquid additives, depending on the requirement each time. Also, this mixing is preferably conducted in homogenizers and/or in the above-mentioned three-roll mill.

The compound according to the invention can be used as a sealing compound and/or as a lubricating compound, each time depending on the thickener or filler that is incorporated. The compound is preferably a paste. A typical application as a sealing compound is its use between the individual mount components in objectives, such as lithography objectives, or in illumination systems of the same. Typical assemblies in optical devices and components, such as objectives and lithography systems, which must be lubricated, are drive elements, such as, e.g., spindle drives, slide guides, screw threads. The compounds according to the invention are also particularly suitable for these applications.

The invention thus also concerns the use of the sealing compounds according to the invention in the production of lenses, prisms, light-conducting rods, optical windows as well as optical components for DUV photolithography, steppers, lasers, in particular, of lasers, wafers, computer chips, as well as integrated circuits and electronic equipment which contains such circuits and chips.

The invention will be explained in more detail in the following example.

EXAMPLE

In order to produce a compound according to the invention in paste form, all volatile components were removed from approximately 2 kg (corresponds to approximately 1 liter) of a perfluorinated polyether (PFPE) in a flash evaporator in high vacuum, whereupon a product was obtained, which has a vapor pressure of <10⁻¹⁴ mbar at room temperature. In this case, the evaporator surface was heated to 150-250° C., preferably 200° C., and a vacuum of <10⁻⁴ mbar, preferably <10⁻⁵ mbar was applied. A cooling trap containing liquid nitrogen as the coolant was disposed between the flash evaporator and the high vacuum pumping stand. As the PFPE, a commercially available product was used, which can be obtained, e.g., from the company Klüber Lubrication under the tradename Tyreno Fluid 18/40V or from Solvay Solexis (Ausimont) under the designation Fomblin Z60. 100 g of boron nitride powder (commercially available powder, Wacker Ceramics, Kempten, with a mean particle size of approximately 5 μm, as indicated in the catalog) per 1000 g of PFPE oil and 100 g of zirconium dioxide with a mean particle size of 0.06 μm were worked into the thus-obtained base oil by slow stirring with a stirring device, and then further homogenized in a three-roll mill.

The thus-obtained paste was spread out on a sample support and introduced into a sample chamber, as is shown in FIG. 1. The sample chamber was irradiated with laser light of a wavelength of 157 μm. In this manner, both the stability of the paste according to the invention under direct irradiation, i.e., by positioning it in the beam path, as well as also under irradiation by scattered light, i.e., by positioning the sample outside the beam path, was investigated. The energy in the case of direct irradiation thus amounted to 0.31-0.42 mJ/cm² per pulse with a number of 62 million laser pulses. The energy in the case of irradiation with scattered light amounted to 0.23-0.34 mJ/cm² per pulse with a number of 150 million laser pulses. Under these conditions, the paste according to the invention showed no decomposition or deposition onto the introduced coated lamellar specimens of CaF₂ when irradiated with scattered light.

In contrast to this, conventional Teflon greases, which contain perfluoro polyethers as the oil base, as well as PTFE powder as the thickener (e.g., which can be obtained under the tradename Barrierta L55/2 from Klüber Lubrication) show an intense brown coloring as well as a decomposition, which deposits on the calcium fluoride lenses. 

1. A compound, comprising a base oil and a powder-form inorganic thickener distributed homogeneously therein the base oil having at room temperature a vapor pressure of <10⁻¹⁰ mbar and the thickener comprising at least one of the group consisting of an inorganic dry lubricant and a ceramic filler with a mean particle size of a maximum of 10 μm.
 2. The compound according to claim 1, wherein said compound is a UV-stable lubricating and/or sealing compound, in particular for clean room applications, for high vacuum and/or optical components.
 3. The compound according to claim 1, the base oil comprising at least one of the group consisting of synthetic liquid perfluoro polyethers and polyphenyl ethers, which are suitable for high vacuum.
 4. The compound according to claim 1, the inorganic dry lubricant comprising at least one of the group consisting of hexagonal boron nitride, graphite, MoS₂, WS₂, NbS₂, TaS₂, AsSbS₄, AsAsS₄, WSe₂, NbSe₂, and TaSe₂.
 5. The compound according to claim 1, further comprising a ceramic filler that is resistant to radiation.
 6. The compound according to claim 5, wherein the filler is a metal oxide.
 7. The compound according claim 1, wherein the filler comprises at least one of the group consisting of ZrO₂, Ta₂O₅, HfO₂, TiO₂, SnO₂, Nb₂O₅, SiO₂, Al₂O₃, ceramic mixed oxide systems, and highly dispersed silicon dioxide.
 8. The compound according claim 1, having a vapor pressure of <10⁻¹² mbar.
 9. The compound according to claim 1, wherein the dry lubricants and/or fillers that are used have a mean particle size diameter of <100 nm.
 10. A method for the production of a UV-stable sealing compound according to claim 1 comprising the steps of: removing volatile components with a vapor pressure of >10⁻¹⁰ mbar from the base oil in high vacuum, dispersing the thickener, yielding a crude paste in the base oil and applying a shearing force to the crude paste in order to pulverize agglomerates of powder particles.
 11. (canceled)
 12. A method for the production of an optical device with components, comprising the step of sealing a space between the components.
 13. The method according to claim 12, wherein said optical device is a lithography objective.
 14. The method according to claim 12, wherein said optical device is a laser.
 15. A compound comprising at least one of the group consisting of synthetic liquid perfluoro polyethers and polyphenyl ethers, and further comprising at least one the group consisting of hexagonal boron nitride, graphite, MoS₂, WS₂, NbS₂, TaS₂, AsSbS₄, As₂S₄, WSe₂, NbSe₂, TaSe₂, ZrO₂, Ta₂O₅, HfO₂, TiO₂, SnO₂, Nb₂O₅, SiO₂, Al₂O₃, ceramic mixed oxide systems, and highly dispersed silicon dioxide. 